Automatic control systems of the electronic, hydraulic and pneumatic types have long been used in industry. However, conventional automatic control systems of the pneumatic or the hydraulic type have many inherent drawbacks, such as: short signal transmittal distance; transit time delays associated with signal transmission; piping being required for signal transmission; few physical variables which can be measured; difficulty in calculating performance; and the high cost of instrumentation, to name but a few. The electronic type of automatic control system can overcome most of these drawbacks. For instance, the signal transmittal distance can be rather long for electronic type systems; such systems have almost no transit time delays; conducting wiring can be used for signal transmission; many physical variables can be measured; performance calculations can easily be made and instrumentation is relative low in cost. These advantageous characteristics, therefore, makes the electronic type of automatic control system most desirable.
However, due to the complexity in wiring of electronic automatic control systems problems frequently occur. Additionally, some of the failures are not predictable and therefore preventative measures are sometimes impossible. Moreover, some troubles are undetectable and thus repair is difficult and the reliable of the system is low. Still further, owing to the difficulties in manufacture, operation and maintenance, the cost of such systems are high. And, since the electric motors generally used in such systems usually have a weak driving force and a low speed, electronic automatic control means cannot be widely adopted.